In order to achieve high-frequency/high-power devices, such as a Gallium Nitride (GaN)-based high electron mobility transistor (HEMT), fine gate electrodes are desired. Also, a variety of research and development are progressing to prevent the increase in cost resulting from the reduction in yield rate caused by breakage in the gate electrode.
Gate electrodes of high frequency/high power devices typically have a T shape or a mushroom shape. The properties of the T-shaped or mushroom-shaped gate electrode can be enhanced by enlarging an upper electrode (gate upper portion) and thinning a lower electrode (gate lower portion).
However, the difference in size between the upper electrode and the lower electrode is increased by thinning the lower electrode. This makes it difficult to deposit a metal to form a gate electrode, and may cause breakage in the gate electrode.
In order to increase the yield rate, a gate electrode has been proposed which has a tapered shape between the upper electrode and the lower electrode.
For example, the following patent documents may be cited:    Japanese Unexamined Patent Application Publication Nos. 2000-58418 and 2000-243758.